Tristable switching in antiferroelectric liquid crystals has been actively investigated because it is expected to be one of the ways to solve some of the essential problems found with conventional surface stabilized ferroelectric liquid crystals (SSFLC). (See A. D. L. Chandani et al.: Jpn. J. Appl. Phys., 27, L729 (1988), A. D. L. Chandani et al.: Jpn. J. Appl. Phys., 28, L1265 (1989) etc.)
The main features of the above mentioned tristable switching include:
(1) The antiferroelectric-ferroelectric phase transition caused by application of an electric voltage, as shown in FIG. 4, has sharp threshold characteristics to a dc voltage. In FIG. 4, the horizontal axis shows the applied voltage and the vertical axis shows the optical transmittance.
(2) As the antiferroelectric-ferroelectric phase transition shows a wide optical hysteresis as shown in FIG. 4, when the antiferroelectric phase or the ferroelectric phase is selected, the selected state can be maintained by applying a biased voltage.
(3) Two states in ferroelectric phase induced by an electric field become optically equivalent.
(4) As the localization of electric impurities in the liquid crystal can be prevented, electrooptical characteristics show no such degeneration with time that is shown with SSFLC.
By the use of these characteristics, time-shared multiplex driving of a simple matrix can be carried out without any duty ratio limitation.
The display principle using an antiferroelectric liquid crystal will be explained by the reference to FIGS. 3A and 3B. As shown in FIG. 3A, the optical axis OA in the antiferroelectric phase is along the smectic layer normal. As shown in FIG. 3B, when a cell comprising a liquid crystal layer 6 maintained between two glass substrates 1 and 2 having transparent electrodes 4, 5 and alignment layer 9, 10, is so positioned between crossed polarizing plates 11, 12 that the optical axis OA was set parallel to either of the polarization axes, the device comes under light shielding condition (tentatively designated as OFF). The numeric character 3 in FIG. 3 indicates a spacer material.
Under this condition, as shown by the hysteresis in FIG. 4, applying a pulsed voltage whose absolute value is equal to or less than .vertline.V(A-F)t.vertline. causes little change in the optical transmittance and the OFF state can be maintained. On the other hand, when a pulsed voltage whose absolute value is equal to or more than .vertline.V(A-F)s.vertline. is applied, transition occurs as shown in FIG. 3A, according to the polarity of the pulse, into a ferroelectric phase (+) having optical axis OF(+) and spontaneous polarization P.sub.S (+), and a ferroelectric phase (-) having optical axis OF(-) and spontaneous polarization P.sub.S (-). As the optical axis makes an angle .THETA. (+) or .THETA. (-) with the polarization axis, the device comes under light transmitting condition (tentatively designated as ON). As the angles .THETA. (+) and .theta. (-) are equal, both phases can be treated as optically equivalent. A liquid crystalline phase showing such character has first been found in 4-(1-methylheptyl oxycarbonyl)phenyl 4'octyloxybiphenyl-4-carboxylate (MHPOBC), and called antiferroelectric chiral smectic C phase (S.sub.CA * phase). Examples of already reported compounds and liquid crystal compositions include those shown in Japanese Patent Laid-Open Publication No. 1-213390.
However, the above mentioned prior art technique has two problems to be solved, described as follows.
One is the problem regarding the stability of the antiferroelectric phase state. Generally, it is said to have sharp threshold characteristics to a direct current voltage, and in the multiplex driving, when the antiferroelectric phase state is once selected, the antiferroelectric phase state can be maintained even if a biased voltage of one polarity is applied. However, it is known that even when an electric field that has its absolute value of equal to or less than the threshold voltage (equal to or less than .vertline.V(A-F)t.vertline. in FIG. 4) is applied to the antiferroelectric phase state, an apparent tilt angle is changed responsive to the field intensity and affects the optical transmittance. (M. Johno et al.: Jpn. J. Appl. Phys. 29 L107 (1990)). This phenomenon is called pretransitional effect in the antiferroelectric ferroelectric phase transition, and lowers the contrast ratio of the device, and is not desirable from the application point of view.
Another problem is that the relaxation speed from the ferroelectric state to the antiferroelectric state is slower than the response speed of the reversed switching, and that the relaxation speed shows temperature dependence. The slow relaxation time leads to the slow response speed in terms of the electric features of the liquid crystal display device and becomes the cause of flickering in the screen. According to the prior art technique, the scanning frequency must be set low to respond to the response characteristics of the liquid crystalline material to be used, that led to a problem that the screen cannot be scrolled smoothly, or the pointing device cannot be moved smoothly.
The present invention solves the above-described problems, and provides liquid crystal compositions and electrooptical devices having high stability and excellent electrooptical characteristics.